Lint all markdown files in CI (#4402)

## Motivation

linkerd/rfc#22

## Solution

Use the [markdown-lint-action](https://github.com/marketplace/actions/markdown-linting-action) to lint all `.md` files for all pull requests
and pushes to master.

This action uses the default rules outlined in [markdownlint
package](https://github.com/DavidAnson/markdownlint/blob/master/doc/Rules.md).

The additional rules are added are explained below:
- Ignore line length lints for code blocks
- Ignore line length lints for tables
- Allow duplicate sub-headers in sibling headers (e.g. allowing multiple ##
  Significant headers in `CHANGES.md` as long as they are part of separate
  release headers)

Signed-off-by: Kevin Leimkuhler <kevin@kleimkuhler.com>

Author: kleimkuhler

.github/workflows/static_checks.yml

@@ -88,3 +88,12 @@ jobs:
         ! -name *.nuspec \
         ! -name *.ps1 \
         | xargs -I {} bin/shellcheck -x -P ./bin {}
+  markdown_lint:
+    name: Markdown lint
+    runs-on: ubuntu-18.04
+    steps:
+    - name: Checkout code
+      # actions/checkout@v2
+      uses: actions/checkout@722adc6
+    - name: Markdown lint
+      run: bin/markdownlint-all

.gitignore

@@ -13,3 +13,4 @@ web/app/yarn-error.log
 **/*.gogen*
 **/*.swp
 charts/**/charts
+package-lock.json

.markdownlint.yaml

@@ -0,0 +1,6 @@
+default: true
+line_length: 
+  code_blocks: false
+  tables: false
+headings:
+  siblings_only: true

bin/markdownlint

@@ -0,0 +1,19 @@
+#!/bin/bash
+
+set -eu
+
+bindir=$( cd "${BASH_SOURCE[0]%/*}" && pwd )
+rootdir=$( cd "$bindir"/.. && pwd )
+markdownlintbin="$rootdir/node_modules/.bin/markdownlint"
+
+markdownlint_version=0.23.1
+
+if [ ! -x "$markdownlintbin" ] || [ "$($markdownlintbin -V)" != $markdownlint_version ]; then
+    if ! [ -x "$(command -v npm)" ]; then
+        echo "Error: npm required to install markdownlint command"
+        exit 1
+    fi
+    npm install markdownlint-cli@$markdownlint_version
+fi
+
+"$markdownlintbin" "$@"

bin/markdownlint-all

@@ -0,0 +1,6 @@
+#!/bin/bash
+
+bindir=$( cd "${BASH_SOURCE[0]%/*}" && pwd )
+
+"$bindir"/markdownlint ./*.md
+"$bindir"/markdownlint ./**/*.md --ignore node_modules/